Earlier aging studies for hydroxy terminated polybutadiene propellants indicated mild to severe hardening or softening during high temperature aging. Further studies indicated that the changes during high temperature aging was attributed to reaction results of the bonding agents.
Prior art bonding agents including MT4 reaction product of 2.0 moles of tris 1(2 methylaziridinyl)phosphine oxide, 0.7 mole adipic acid, and 0.3 mole tartaric acid), HX752 (bis isophthaloyl 1(2 methyl) aziridine), and other type bonding agents have been employed individually in aging studies to determine effects on propellant physical properties. A hydroxy terminated propellant composition cured with a diisocyanate curing agent and containing MT4 as the bonding agent showed significant softening at high temperature (52.degree. F. and 70.degree. C.) aging. A propellant containing HX752 and cured with isophorone diisocyanate hardened severely under the same conditions.
Applicant's coinvention with Henry C. Allen filed as Ser. No. 840,927 on Oct. 11, 1977, issued as U.S. Pat. No. 4,090,893, and assigned to The United States of America as represented by the Secretary of the Army, Washington, D.C., teaches and claims a bonding agent system for use in a hydroxy terminate polybutadiene propellant composition that is curable with a diisocyanate curing agent. This propellant contains a high solids loading of aluminum metal fuel and ammonium perchlorate oxidizer, and it employs a bonding agent system consisting of MT4, the reaction product of 2.0 moles of tris 1 (2 methyl aziridinyl) phosphine oxide, 0.7 mole adipic acid, and 0.3 mole tartaric acid in an amount from about 0.10 to about 0.20 weight percent of the propellant composition; HX752, bis isophthaloyl 1(2 methyl) aziridine in an amount from about 0.10 to about 0.20 weight percent of the propellant composition; and BIDE, butyliminodiethanol in an amount from about 0.02 to about 0.05 weight percent of the propellant composition.
The above hydroxy-terminated polybutadiene propellant composition employed an optional delay quick cure catalyst system (of each parts) of triphenylbismuthine, magnesium oxide Mg0 and maleic anhydride (MAN)(0-0.05% each). The delayed quick cured system with isophorone diisocyanate (IPDI) makes possible a longer pot like but a shorter cure time of only 2 days as compared to more than 5 days for propellants cured with dimeryl diisocyanate (DDI) and 14 days for propellants cured with IPDI without using a catalyzed cure.
Applicant's U.S. Pat. No. 4,913,753 teaches that an amount from about 0.5% to about 2.0% weight percent of meta-tetramethylxylene diisocyanate (m-TMXDI) is effective in extending propellant pot life and yielding a propellant composition which is less sensitive to temperature change as compared with an isophorone dissocyanate (IPDI) cured composite rocket propellant composition. The domestically produced m-TMXDI is made by a non-phosgenation process developed by American Cyanamid Company's research laboratories. The original proposed application, prior to applicant's patent, for m-TMXDI and p-TMXDI was in reaction injection molding (RIM) which permits the manufacture of urethane parts which require no post-painting. The isomers of either TMXDI which are naturally white (or can be pigmented any desired color) are light stable; and if scratched or damaged in use, they retain their integral color. Triphenylbismuthine (TPB) has been used singly in propellants to obtain higher modulus and/or stress values in composite propellants with isocyanate cured hydroxy terminated polymer binders. TPB has been widely accepted for the designated benefits it provide; however, since the cost of TPB is in the range of $1.08 per gram, an adjunct to TPB having only a fractional part of the cost of TPB and which imparts equal or improved benefits to the mechanical properties in hydroxyterminated binders would be attractive in view of present budget restraints to reduce cost.